As well known to those skilled in the art, typical bicycles, preferably used for leisure sport, short-distance locomotion, etc., individually comprise a pipe frame with both a saddle and a handlebar. In such a bicycle, front and rear wheels are rotatably held by two forks at the front and rear ends of the frame with shafts and bearings. The frame also holds a one-way drive device comprising two sprockets: a large-diameter drive sprocket and a small-diameter ratchet sprocket. The drive sprocket has a shaft, which is provided with two pedals at both ends and is rotatably fitted into the fixed bush of the frame. The ratchet sprocket is held by the frame at the center of the rear wheel and is connected to the drive sprocket through an endless chain. The ratchet sprocket is provided with a ratchet mechanism or a one-way clutch for exclusively transmitting the normal rotating force of the drive sprocket to the shaft of the rear wheel. That is, when a user, sitting on the saddle and gripping the handlebar with both hands, pedals to rotate the drive sprocket in a normal direction, the normal rotating force of the drive sprocket is transmitted to the ratchet sprocket through the chain prior to being transmitted to the rear wheel, thus rotating the rear wheel and making the bicycle go forward on the ground. However, when the drive sprocket is rotated in a reversed direction, the one-way clutch of the ratchet sprocket does not transmit the reversed rotating force of the drive sprocket to the rear wheel, but allows the ratchet sprocket to be run idle. In such a case, the bicycle inertially goes forward for a time.
However, such a one-way drive device for bicycles is problematic in that the drive sprocket has to be continuously rotated in the normal direction when the bicycle requires going forward. It is thus necessary for a user to continuously pedal the bicycle forward while riding on the bicycle. Therefore, in the case of a long-distance bicycle trip, both legs of the user suffer from tension, causing the user to feel stiff in the legs and preventing the leg muscles from being uniformly developed and reducing the muscular motion effect while riding on the bicycle. In addition, when the user dismounts from the bicycle and walks on the ground after a long-time pedaling, the leg muscles, which have tension due to the unidirectional pedaling, fail to directly adapt themselves to walking thus causing the user to stagger on the ground. In this regard, a user sometimes stops the pedalling of a bicycle or pedals the bicycle in a reversed direction. However, such a reversed rotating force of the drive sprocket is not transmitted to the rear wheel due to the one-way clutch and results in a consumption of user's pedalling force.
In an effort to overcome the above problems caused by such a one-way drive device for bicycles, Korean Patent Publication No. 95-13589 discloses a reversible drive device for bicycles, which is capable of making a bicycle to go forward on the ground by the reversed rotating force of a pedal shaft in addition to the normal rotating force. The above Korean reversible drive device effectively transmits the rotating force of a pedal shaft to the rear wheel regardless of the rotating direction of the shaft, thereby making the bicycle go forward when a user rotates the pedal shaft in either direction. The above reversible drive device comprises a frame, which is provided with a mount bracket for holding a drive sprocket on the frame. The drive device also has a drive power transmission unit a power conversion unit and a switch lever unit. The power transmission unit is used for transmitting the rotating force of a pedal shaft to the drive sprocket. The power conversion unit is selectively brought into detachable engagement with the power transmission unit, thus changing a reversed rotating force of the pedal shaft into a normal rotating force prior to transmitting the rotating force to the drive sprocket. The switch lever unit is selectively operated by a user to change the drive mode of the reversible drive device between two modes: a normal pedalling mode and a reversed pedalling mode. In the normal pedalling mode, the power transmission unit is free from the power conversion unit, so the power transmission unit transmits the normal rotating force of the pedal shaft to the drive sprocket. In the reversed pedalling mode, the power conversion unit engages with the power transmission unit, thus changing the reversed rotating force of the pedal shaft into a normal rotating force prior to transmitting the rotating force to the drive sprocket.
That is, when the switch lever unit is operated by a user to change the drive mode into the reversed pedalling mode, the power conversion unit is brought into engagement with the power transmission unit, so the reversed rotating force of the pedal shaft is changed into the normal rotating force, thus rotating the power transmission unit in a normal direction. Therefore, the above reversible drive device allows a bicycle to go forward regardless of the rotating direction of the pedal shaft.
However, the above reversible drive device for bicycles is problematic in that when a user wants to change the drive mode of the drive device between the two modes while riding on a bicycle, the user has to operate the switch lever unit. Therefore, the reversible drive device is inconvenient to the user.
The reversible drive device is also designed so that it reduces the speed ratio during the reversed pedalling mode where the rotating force of the pedal shaft is transmitted to the power transmission unit through the power conversion unit. Therefore, the reversible drive device reduces the speed of a bicycle during the reversed pedalling made in comparison with the normal pedalling mode. Another problem experienced in the above reversible drive device is that the power conversion unit and the power transmission unit individually have a complex construction and comprise a plurality of elements, thus complicating the process of producing bicycles. Therefore, the above reversible drive device reduces productivity and increases production cost of bicycles.